Field of the Invention.
Ion cyclotron resonance (ICR) is a known phenomenon and has been employed in the context of mass spectroscopy. Essentially, this mass spectrometer technique has involved the formation of ions and their confinement within a cell for excitation. Ion excitation may then be detected for spectral evaluation.
Ion formation, trapping, excitation and detection, in the environment of mass spectroscopy, are known techniques. For example, U.S. Pat. No. 3,742,212 issued June 26, 1973 to McIver discloses an Ion Cyclotron Resonance Mass Spectrometer employing these techniques. An improvement to the noted patent is disclosed in U.S. Pat. No. 3,937,955 issued Feb. 10, 1976 to Comisarow and Marshall and which is commonly designated as a Fourier Transform Mass Spectrometer. Both of the noted patents are hereby incorporated by reference. Also incorporated by reference is U.S. Pat. No. 4,581,533 issued Apr. 8, 1986 to Littlejohn and Ghaderi and which is commonly owned with the present invention.
A mass spectrometer of the type disclosed in the above incorporated patents is illustrated diagramatically in FIG. 1. In FIG. 1, a superconducting, solenoidal magnet 10 surrounds a vacuum chamber 11 while a pump 12 is connected to the vacuum chamber 11 to establish high vacuum conditions in known manner. Magnet 10 establishes a magnetic field through the vacuum chamber including a region along the geometric central axis of the magnet at which the field is high in intensity and homogeneity and wherein the magnetic flux lines are generally parallel to the central axis. A sample cell 13 is positioned at or within this region, in known manner. The arrow designated B indicates the direction of the field established by the magnet 10, at least through the region occupied by the sample cell 13.
A sample to be analyzed is introduced into the sample cell 13 via substance connections 14. An electron gun 15 is connected to a suitable power supply by electrical connections 16. Connections 14 and 16 are known in the art and are not described in detail herein. The electron beam emitted by the electron gun 15 passes through apertures in the end (trapping) plates of the sample cell 13 to impinge on a collector 17. Within the cell 13, the electron beam forms ions, in known manner.
Mass spectrometers of the prior art have been known to have problems of sensitivity, resolution and exact mass measurement. Most attempts to resolve these problems have centered around the design of the ion analyzer or sample cell--cell 13 in FIG. 1. Indeed, the disclosure of the last filed of the incorporated specifications includes an improvement in the analyzer or sample cell.
So as to take full advantage of the cell dimensions, it is important that the ions be formed in the cell at the cell center and at the center of the magnetic field. In the prior art, this has been accomplished by positioning the cell at the center of the magnetic flux lines and by positioning the electron gun 15 such that the electron beam travels along what is commonly referred to as the Z axis--the axis that is the geometrical center of the solenoidal magnet 10. It has also been the practice to position the electron gun 15 within the magnet 10 close to the cell 13. The practice has complicated the servicing of the electron gun 15 in that it is located deep inside the vacuum chamber 11 and magnet 10 and often requires the removal of the cell 13 as well. In addition, the proximity of the electron gun 15 to the cell 13 has resulted in an introduction of electrical noise into the cell 13 and interference with the detection system.
In addition to the above, the position of the electron gun on the Z axis effectively occupies the Z axis and prevents the use of an alternative ionizing device at that location. Other ionizing sources may have similar considerations to those mentioned above.